1. Field of the Invention
The present invention relates to a process for preparing a binder resin used in a toner which is used in a dry developer used for an image forming process such as electrophotography, electrostatic recording or magnetic recording.
2. Related Background Art
A large number of methods have been conventionally known as electrophotography, as disclosed in U.S. Pat. No. 2.297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member, utilizing a photoconductive material and according to various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. In the case when the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on a photosensitive member.
As developing processes in which an electrostatic latent image is formed into a visible image by the use of a toner, known methods include the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these development processes, fine powder obtained by dispersing a dye and/or pigment in a natural or synthetic resin has been hitherto used. For example, particles formed by finely grinding a binder resin such as polystyrene comprising a colorant dispersed therein, to have a size of about 1 to 30.mu. are used as the toner. A toner incorporated with magnetic material particles such as magnetite is also used as the magnetic toner. On the other hand, in a system in which a two-component type developer is used, the toner is usually used by mixture with carrier particles such as glass beads, ion powder and ferrite particles.
Nowadays, such recording processes have been widely utilized not only in commonly available copying machines, but also in printers that output information from a computer, or for the printing of microfilms. Accordingly, a higher performance has become required, and the above recording processes have now been required to simultaneously achieve the improvements in performance such that an apparatus is made more small-sized, more lightweight, more low energy, more high-speed, more maintenance-free, and more personal. In order to meet these requirements, the needs on toners have become severer in various aspects. For example, when the copying machines or printers are made small-sized, heat sources such as heat-fixing units and exposure lamps are squeezed into a narrow space, so that the temperature inside the machine tends to be higher. Hence, toners must be made to have an improved blocking resistance. In order to make the copying machines or printers lightweight, a fixing roller is so designed as to be more thin-walled and slender, and a cleaning mechanism of the fixing roller or a cleaning mechanism of a photosensitive member ends to be more simple and lightweight. Thus, there is a tendency that the machine is provided with no applicator used for applying an anti-offset oil to a fixing unit. This makes it necessary to improve fixing properties of toners, offset resistance thereof, and cleaning resistance of photosensitive members. In order to make the copying machines or printers more energy efficient (specifically stated, to make them consume less power) or to make them more high-speed, the fixing properties of toners must be improved. In addition, in order to make the copying machines or printers more personal, the reliability must be improved, and it becomes important to eliminate paper jam. The paper jam may commonly often occur when a transfer sheet e.g., copy paper) winds around a roller, and thus it becomes necessary for toners to be capable of suppressing the winding of paper around a fixing roller. However, as shown in FIG. 6, the performance required when a toner is prepared and the properties of a toner itself often conflict with each other.
The needs on toners are severe as will be seen from the above examples, and it is difficult to meet the requirements unless these performance features and properties are simultaneously improved. However, it would be ridiculous if the achievement of these improvements results in lowering of development performance such as image quality and durability of toners, and production efficiency of toners.
These greatly depend on the performance of the binder resin used in toners. It has been proposed to improve the characteristics of a toner by the use of a release agent, a plasticizer or other additives. Use of these, however, is a supplementary means.
Various methods have been proposed for the improvement of binder resins used in toners.
For example, Japanese Patent Application Laid-Open No. 6-158340 (corresponding to British Patent No. 2,078,385) proposes a toner containing a binder resin comprised of a low-molecular polymer and a high-molecular polymer. In order to improve the offset resistance of toners, it is necessary to make larger the molecular weight of the high-molecular polymer or to increase the proportion of the high-molecular polymer. In such an instance, the grindability of a toner is so extremely lowered that it is difficult to obtain a product satisfactory from a practical viewpoint.
Japanese Patent Application Laid-open No. 58-86558 proposes a toner blended with a polymer cross-linked with a low-molecular polymer, which is a toner comprising a low-molecular polymer and an insoluble infusible high-molecular polymer as main resin components. In this toner, the fixing properties and grindability are presumed to be improved. However, it is difficult to satisfy both the offset resistance and grindability in a high performance, because the weight average molecular weight/number average molecular weight (Mw/Mn) of the low-molecular polymer is as small as not more than 3.5 and the proportion of the insoluble infusible high-molecular polymer is as large as from 40 to 90 wt.%. Thus, it is difficult from a practical viewpoint to give a toner satisfactory the fixing properties, offset resistance and grindability unless a fixing machine is provided with an apparatus for feeding an anti-offset liquid. Moreover, the toner must be heat-kneaded at a temperature far higher than that in usual instances or heat-kneaded at a high shear, because the insoluble infusible high-molecular polymer used in a larger amount turns out to have a very high melt viscosity as a result of the heat-kneading carried out when the toner is prepared. In the instance where it is kneaded at a high temperature, the toner characteristics tend to be lowered because of thermal decomposition of other additives. In the instance where it is kneaded at a high shear, the molecules of the binder resin may be excessively cut. Thus, there is the problem that the desired offset resistance can be achieved with difficulty.
Japanese Patent Application Laid-open No. 60-66958 proposes a toner comprising a resin composition obtained by polymerizing monomers in the presence of a low-molecular eight poly(.alpha.-methylstyrene) having a number average molecular weight of from 500 to 1,500. In particular, this publication discloses that the number average molecular weight Mn may preferably range from 9,000 to 30,000. With an increase in Mn for the purpose of improving offset resistance, the fixing properties of the toner and the grindability at the time the toner is prepared may become more questionable from a practical viewpoint, so that it is difficult to satisfy a good offset resistance of the toner and the grindability at the time the toner is prepared. Thus, the toner having a poor grindability at the time the toner is prepared brings about a lowering of production efficiency, and also coarse toner particles tend to be mixed into the toner, undesirably resulting in black spots around a toner image.
Japanese Patent Application Laid-open No. 56-16144 (corresponding to U.S. Pat. No. 4,499,168) proposes a toner containing a binder resin component having at least one maximum value in each region of a molecular weight of from 10.sup.3 to 8.times.10.sup.4 and a molecular weight of from 10.sup.5 to 2.times.10.sup.6, in the molecular weight distribution measured by GPC. This toner can give a superiority in the grindability at the time the toner is prepared, offset resistance of the toner, fixing properties, anti-filming or -fusing to a photosensitive member, image quality, etc. The toner, however, is sought to be further improved in the offset resistance and fixing properties.
Japanese Patent Application Laid-open No. 63-223014 (corresponding to European Patent Publication No. 259,819) proposes a binder resin used in a toner, capable of improving the fixing properties of the toner, and also answering the recent severe needs while retaining or improving other various performances. This binder resin contains from 10 to 70% by weight of a THF-insoluble matter, has at least one peak in the region of a molecular weight of from 2,000 to 10,000 and at least one peak or shoulder in the region of a molecular weight of from 15,000 to 100,000, in the molecular eight distribution measured by GPC of a THF-soluble matter, and contains a component with a molecular weight of not more than 10,000, in an amount of from 10 to 50% by weight based on the resin.
However, it is strongly sought to improve the binder resin to improve the development performance of the toner, in particular, the environmental stability, duration stability and so forth so that additional various demands can be met.
Japanese Patent Application Laid-open No. 56-8416 proposes a process for preparing a resin composition comprised of a high-molecular polymer and a low-molecular polymer, comprising the steps of producing a high-molecular polymer by polymerizing monomers without use of any polymerization initiator, dissolving the resulting high-molecular polymer in monomers, and producing a low-molecular polymer by polymerizing monomers in the presence of a polymerization initiator. The toner in which the above resin composition is used does not exhibit preferred environment stability because of the presence of polymerization initiator fragments at many terminals of the low-molecular polymer.